1. Field of the Invention
The present invention relates to a semiconductor device manufacturing apparatus for heat processing of semiconductor wafers and a method of controlling the semiconductor device manufacturing apparatus.
2. Description of the Related Art
FIG. 8 is a schematic view of a conventional semiconductor device manufacturing apparatus, for example, a diffusion furnace. In FIG. 8, a heater 2 is disposed on the outside of a cylindrical processing chamber 1 made of quartz. The cylindrical processing chamber 1 has an opening at its lower end. An upper portion of the processing chamber 1 is provided with a gas introducing port 4. Within the processing chamber 1, a plurality of semiconductor wafers are supported on, for example, a boat 5 made of quartz. The boat 5 is placed on a boat base 6, which is connected to a boat loader 7. The boat 5 is fed in and out of the processing chamber 1 by the vertical movement of the boat loader 7.
The conventional semiconductor device manufacturing apparatus arranged in the manner described above is operated as follows: first, the semiconductor wafers 3 to be heated are placed on the boat 5, and that boat is placed in the processing chamber 1 by means of the boat loader 7. Thereafter, the semiconductor wafers 3 are heated by the heater 2 while a gas, such as oxygen or nitrogen, is supplied from the gas introducting port 4, when necessary. After heating has been conducted for a desired period of time and at a desired temperature, the boat 5 is pulled out of the processing chamber 1 by means of the boat loader 7.
Generally, the semiconductor wafers 3 are pulled out of the processing chamber 1 at a very high temperature ranging from 800.degree. C. to 900.degree. C., because it takes long time for the temperature of the diffusion furnace to decrease due to the large heat capacity thereof and pulling out of the semiconductor wafers 3 at a low temperature lengthens the processing time and thus reduces productivity.
Once pulled out of the high temperature processing chamber 1, the semiconductor wafers 3 begin to cool starting from the periphery thereof. FIG. 9 is a graph showing how the temperature of the wafers behaves at various temperatures of the processing chamber 1 when the wafers are pulled out of the chamber. The abscissa represents the temperature of the center of the semiconductor wafer 3, and the ordinate represents the difference in the temperature between the center of and the periphery of the semiconductor wafer 3. In the graph, curves A, B, and C respectively show the behavior of the temperature of the semiconductor wafers 3 when the wafers are pulled out of the chamber 1 at 800.degree. C., 850.degree. C. and 900.degree. C. As can be seen from the graph, there is a difference in the temperature between the center and the periphery of the semiconductor wafer 3. That temperature difference generates stress. When the temperature of the processing chamber 1 is high and the temperature difference within the semiconductor wafer 3 is large, crystalline defects or dislocation occur to a degree which cannot be repaired. The region where defects or dislocation occur is represented in FIG. 9 by area D. The graph shown in FIG. 9 is just an example, and the temperature difference increases where the semiconductor wafer 3 has a larger diameter or where the pulling rate of the semiconductor wafers 3 is higher.
In the semiconductor device manufacturing apparatus of the type described above, the difference in the temperature between the center and periphery of the semiconductor wafer 3 is great when the semiconductor wafer is pulled out of the processing chamber 1, generating crystalline defects or dislocations and thereby adversely affecting the quality of the semiconductor wafer 3.